1. Field of the Invention
This invention relates to power line switch devices in general and more particularly to time delay power line switch devices.
2. Brief Description of the Prior Art
Power line switches are very well known devices. An ordinary lamp switch is an example of a two state, on/off, power line switch. Such switches are generally placed in the desired on or off state manually.
Other switches are known which eliminate the need for manual interaction to cut power. These automatic devices typically cut power off from a load at the expiration of a predetermined time period. A combination of a capacitive element, with known discharge characteristics, and a number of transistors, with known cut-off characteristics, is typically employed to realize an automatic switch. The transistors in such a device are controlled by the amount of charge on the capacitive element over time. While the capacitive element is in the process of, for example, being discharged according to its known characteristics, the transistors may be maintained in their conductive state (usually associated with the on state of the switch) permitting the load to receive power from the power source. The cut-off characteristics of the transistors and the discharge characteristics of the capacitor combined effectively determine a time interval, variable by the switch designer, for maintaining the switch in its on state.
At some point in time the charge on the capacitor causes transistor cut-off placing the transistors in their nonconductive state (usually associated with the off state of the switch) to effectively prevent power from reaching the load. Thus the switch automatically enters its off state.
Examples of automatic cut-off type switches employing a capacitive discharge type time delay may be seen in Smith, U.S. Pat. No. 3,480,801, issued Sept. 27, 1965; Prapis, U.S. Pat. No. 3,210,613, issued Oct. 5, 1965; Yello, U.S. Pat. No. 3,509,361, issued Apr. 28, 1970; and Mitchell, U.S. Pat. No. 3,621,276, issued Nov. 16, 1971.
Other known prior art devices attempt to accomplish automatic time delay power cut-off switching with a minimal number of switch components. Obviously, it is desirable from both economic and reliability standpoints to minimize the number of components in a switch. Nellis, U.S. Pat. No. 3,033,998, issued May 8, 1962 teaches such a minimal configuration utilizing only two transistors in combination with a capacitor. The Nellis switch is normally de-energized with both transistors non-conducting. When a relatively short duration pulse is supplied to the switch the two transistors are driven into saturation and supply current to the load through one of the transistors. The Nellis switch has a timing circuit (which includes the capacitor) for maintaining the switch in the on state for a predetermined period.
All of the known prior art time delay switches draw current from the power source when the circuit is inactive, i.e., when the switch is in the off state. This continuous power drain results in frequently having to rejuvinate or replace the power source, typically a battery.
In addition to not being power conservative, the known minimal configuration time delay cut-off switches are capable of remaining in their on state for only short peirods of time. This is due to the fact that the known switches employ bipolar transistors having low input impedances to define the time interval over which the switch remains in the on state.
In light of these problems, it is an object of the invention to provide a power conservative time delay cut-off switch which eliminates the need for frequent rejuvination or replacement of the power source due to the drain placed on the power source by known switching devices.
It is a further object of the invention to provide a power conservative time delay cut-off switch having a minimum number of components.
Still further, it is an object of the invention to provide a power conservative time delay cut-off switch which is capable of operating in its on state over relatively long periods of time.